The new advent of power batteries, in particular Li-ion with nanotitanate, exhibiting a very low internal impedance, a very good resistance to deep discharges and a life of thousands of cycles, is conducive to a strong need for quick chargers. It is now possible to charge a vehicle in 5 minutes. For example, buses with a minimal amount of on-board batteries may autonomously run about twenty kilometers and recharge in a few minutes at the terminus.
A problem resides in the economical construction of a charger that may handle the power levels required by such recharging speeds. The power involved is in the order of hundreds of kilowatts and even megawatts in the case of trains and buses.
Not only the charger must handle these power levels, but the connection points compatible with the electrical network are limited or expensive. In the case of a bus station, the power subscribed from the network may be established as function of the peak period (which corresponds to the maximum billable power), but the rest of the time, consumption may be much lower. An extreme case is in a residential application where the charger is intended to be used for some 10 minutes per day to fill up an electric vehicle e.g. at 100 kilowatts.
There is thus a need for a very powerful charger that has an internal storage intended to smooth the power seen by the electrical network.
A classical solution consists in using a small charger that operates over long time periods, preferably during the low energy demand hours, in order to store the energy in a local intermediate battery, also of a power type. When a vehicle is connected for a quick charging, a powerful DC-DC converter is used to transfer a big energy block from the intermediate battery to that of the vehicle. This solution solves the problem of the utilization factor, but always requires a large size converter.
Furthermore, the intermediate battery requires a load balancing system between the various cells and modules that form it. This system adds up to the costs and even possibly to the energy losses of the whole.
Also, the multiplication of chargers with power electronics is likely to impair the quality of the electric wave of the network. The harmonics and interferences generated by chopped waveforms are the main responsible ones. The standards on the apparatuses of that type may only evolve by tightening the electromagnetic emission criteria. There is thus a need for improvement of the current waveform quality of such chargers.
For this purpose, the unitary power factor power converters with sinusoidal pulse width modulation (sinusoidal PWM or SPWM) form a technically attractive solution although high modulation frequencies must be used, which causes switching losses. A multi-level voltage variant allows reducing the switching frequency while preserving the waveform quality. However, the supply and holding of continuous voltages having several levels on the DC side cause control and/or complexity problems.
Finally, there is a growing demand for network-connected apparatuses to be capable of providing assistance to the network in certain circumstances. Be it for power smoothing (i.e. contributing to some extent to provide power during peak periods), voltage support by reactive power supply or even harmonic filtering. Thus, a charger that would satisfy these needs is an advantage.